User blog comment:TrickyTuna/The Gym Logic/@comment-26120508-20170611032257

Dark is weak to Fighting, Fairy, and Bug

Fighting is weak to Fairy, Flying, and Psychic

Fairy is weak to Steel and Poison

Bug is weak to Fire, Flying, and Rock

We can eliminate Fire and Flying because there are already gyms for them. Rock is highly unlikely because we already have a Ground-type gym.

So the 8th gym could be Steel, Poison, Psychic, or Fairy (unless the PBB devs troll us again)